The present invention relates to high intensity discharge (HID) lamps and, more particularly, to an HID strobe lamp and a method for operating an HID strobe lamp.
Light is produced by high intensity discharge (HID) lamps when an electric current arcs between two closely spaced electrodes in a sealed quartz-glass or ceramic capsule, known as a discharge tube, arc tube or burner, containing a vapor of metal and gas. Free electrons in the arc collide with the metal atoms in the vapor exciting electrons of the metal atoms to a higher energy state. When the excited electrons return to their original, lower, energy level, electromagnetic radiation is emitted having a wavelength determined by the energy level of the electrons and the constituency of the vapor in the capsule. Compared to a filament-type halogen lamp, HID lighting typically produces light more efficiently and with a color temperature more closely approximating that of sunlight. As a result, HID lighting has been adapted to many residential and commercial uses, including building, street and sports arena lighting, automotive lighting and aviation lighting. Despite the advantages of HID lighting, the operating characteristics of HID lamps detract from the usefulness of the lamps as stroboscopic light sources.
Unlike an incandescent lamp, an HID lamp does not immediately illuminate when the power is supplied to the lamp. When a voltage is applied to the electrodes of an HID lamp, there is an initial delay while the gas is heated during which the lamp acts as an open circuit. Following the initial delay, the gas in the discharge tube is ionized with a high voltage pulse, commonly 2-20 kilovolts (kV), enabling the ionized gas to conduct a relatively high current between the lamp's electrodes and producing a glow discharge. Following initiation of the glow discharge, the lamp requires a relatively high current for a short period of time to sustain the current flow between the electrodes as the electrodes begin to warm-up. The electrodes must be warmed up so that they can supply sufficient numbers of electrons to sustain an arc and, if the warm-up period is not adequate, the lamp may extinguish later in the start up process. Following warm-up, a run-up phase during which the electrodes continue to heat up and the voltage applied to the lamp is increased precedes steady-state operation of the lamp. HID lamps are typically operated with an alternating current of 45-225 v and a frequency commonly in the range of 150-400 Hertz (Hz). The alternating polarity of the current reduces loss of material from the electrodes and the relatively low frequency avoids acoustic resonance which can cause the lamp to flicker, extinguish or even self-destruct. Depending upon the type of lamp and its condition, an HID lamp may require from 10 seconds to 15 minutes to reach steady state operation and full light output from a cold start.
While initiating operation of an HID lamp takes considerable time, the time required to restrike (restart) an extinguished lamp is typically longer and commonly twice as long as the cold start up interval. If an extinguished HID lamp is not allowed cool adequately before a restrike is attempted, the arc may extinguish before steady state operation can be attained. Some HID lamps have a “hot restrike” capability enabling restriking of the lamp before the lamp has cooled. Hot restriking typically requires an electrical pulse with 10 times the voltage required for a cold start of the lamp and may adversely effect the lamp's life. While, the output of an HID lamp can be dimmed by reducing the power to the lamp, reducing the power and the output of the lamp to less than about 60% of rated levels runs the risk that the lamp may extinguish requiring a restrike. As the input power to the lamp is reduced to dim the output, the discharge arc becomes unstable with each change in polarity of the alternating current producing undesirable electromagnetic interference and, if the energy flowing in the lamp is no longer sufficient to maintain the electrode temperature above the thermal electron emission temperature, the arc may extinguish. Many strobe light applications require a lamp output differential that is greater than the output differential (approximately 40%) attainable by reducing the magnitude of the power to the lamp and the lengthy restrike interval and/or high restrike voltage requirements make HID lamps generally unsuitable for use as stroboscopic light sources.
What is desired, therefore, is a system and method for operating an HID lamp that enables substantially reducing the light output of the lamp for an extended period and rapidly returning the lamp to full illumination.